Device for controlling pumps for operating hydraulic motors equipment

ABSTRACT

A device for controlling pumps for operating hydraulic motors comprising at least two variable displacement pumps, valve means for each said pump disposed between the discharge side of the pump and the hydraulic motors for switching a charge of hydraulic fluid delivered to the hydraulic motors upon operation of an operating lever, and a hydraulic servomechanism for each said pump operated by a control lever coupled to said operation lever for moving a discharge flow control member of the pump, said servomechanism having a control circuit including a first control valve operated by said control lever for forming a flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the amount of operation of the control lever, and a second control valve responding to the discharge pressure of the pump for forming another flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the discharge pressure of the pump.

United States Patent [72] Inventors Katsuro Abe Matsudo-shi; Saburo Kakinuma, Kashiwa-shi; Masaaki Uno, Tokyo, Japan [21] Appl. No. 830,940 [22] Filed June 6, 1969 [45] Patented May 25, 1971 [73] Assignee Hitachi, Ltd.

Tokyo, Japan [32] Priority Sept. 6, 1968 3 I J p [31] 43/63756 [54] DEVICE FOR CONTROLLING PUMPS FOR OPERATING HYDRAULIC MOTORS 3 Claims, 14 Drawing Figs.

[52] US. Cl 60/52 [51 Int. Cl [50] Field of Search 60/52 (RE), 52 (VSP), 19

[5 6] References Cited UNITED STATES PATENTS 2,316,926 4/1943 Willett 60/52VSPUX YAN 3,161,245 12/1964 Thoma 60/19X 3,323,607 6/l967 Futamata 60/52VSPUX Primary Examiner-Edgar W. Geoghegan AttorneyCraig, Antonelli, Stewart & Hill ABSTRACT: A device for controlling pumps for operating hydraulic motors comprising at least two variable displacement pumps, valve means for each said pump disposed between the discharge side of the pump and the hydraulic motors for switching a charge of hydraulic fluid delivered to the hydraulic motors upon operation of an operating lever, and a hydraulic servomechanism for each said pump operated by a control lever coupled to said operation lever for moving a discharge flow control member of the pump, said servomechanism having a control circuit including a first control valve operated by said control lever for forming a flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the amount of operation of the control lever, and a second control valve responding to the discharge pressure of the pump for forming another flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the discharge pressure of the pump.

PATENTED M2 19 3579 .986

' SHEET 01 (1F 1o INVENTORS KATSuKO AB 548M120 KAKINuMA and MASAAKI 4N0 ATTORNEYS PATENTED HAY25 1971 3579,9855

sum 02 or 10 T FIG. 4 I5 I I iii llml INVENTOR$ KATSHRO ABE, SABHRO AAKINHMA and MASAAKI NO Z V M ATTORNEYS PATENTEU-mw 2 5 I97! sum 03 0F 10 FIG. 5

INVENTORS snrauno KAKZNH MA and KATSURO ABE MASAAKIZ MNO W M4 9M ATTORNEYS PATENTEU uAYzslsn 3; 579,986

sum 05 or 10 I INVENTORS KATSMKO ABE, SABMR KAKZNHMA a y MASAAKI Na BYL47M,M w

ATTORNEY PATENTEUmzsmn 3.579.986

saw 05 0F 10 (2 F/G. a

INVENTORS KATsuRo ABE. sABuRo KAKINuMA and MASAAKL unto ZMA'Z, M J -M M ATTORNEY PATENTED HAYZS I97] SHEET 07 0F 10 FIG. 9

I INVENTORS saauno NAKINHMA 4nd KATSuRo 3 MASAAKI N0 film m (M11 ATTORNEYS 'PATENTEDHAYZSIBYI 3579.986

sum as or 10 (2) F/G. I0

INVENTOR'S KATSuRO M313. SABHRO KAKuvu/vm a MASAAKI N ATTOR N EY.

PATENT-EB was lsn sum as or 1o INVENTOR$ S'AfiHRD KAKILNMMA and KATsuRo ABE,

MASAA KI uNo M 46mm 9M1 PATENTED was an sum 10 0F 1o FIG. 12

INVENTOR$ .SAGuKo KAKIIvuMA J KATS um: ABE.

ATTORN EYS DEVICE FOR CONTROLLING PUMPS FOR OPERATING HYDRAULIC MOTORS The present invention relates to devices for controlling pumps for operating hydraulic motors. In particular, the invention deals with a control device having particular utility in hydraulic apparatus, such as hydraulic shovels, in which a plurality of hydraulic motors are operated by at least two variable displacement pumps driven by a single prime mover.

In general a hydraulic shovel is provided with a number of hydraulic motors such as hydrocylinders, hydraulic motors and the like and so designed that more than two of said hydraulic motors are operated at a time in order to increase operation efficiency. A charge of hydraulic fluid under pressure is often delivered to such hydraulic equipments by at least two variable displacement pumps driven by a single prime mover, In such operation, it is required to maintain the total output power of these pumps at a level below the maximum output power of the prime mover.

The object of the present invention is to provide a mechanism for controlling each pump in the hydraulic apparatus constructed as aforementioned such that the output power of the prime mover can be maintained at a level which is nearly 100 percent of its maximum output power in order to operate the pumps most economically.

According to the invention, a device for controlling pumps for operating hydraulic motors comprises at least two variable displacement pumps, valve means for each said pump disposed between the discharge side of the pump and the hydraulic motors for switching a charge for hydraulic fluid delivered to the hydraulic motors upon operation of an operation lever, and a hydraulic servomechanism for each said pump operated by a control lever coupled to said operation lever for moving a discharge flow control member of the pump, said operation lever for said valve means and said control lever for said hydraulic servomechanism being operatively connected to each other such that when the former is in its maximum stroke position the latter is in a position in which the discharge flow of the pump is maximized and when the former is in its neutral position the latter is in a position in which the discharge flow of the pump is minimized, said hydraulic servomechanism having a control circuit including a first control valve operated by said control lever for forming a flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the amount of operation of the control lever and a second control valve responding to the discharge pressure of the pump for forming another flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the discharge pressure of the pump, said first control valve including a movable member operatively connected to said control member of the pump for stopping said flow of hydraulic fluid when said control member has been moved to said position corresponding to the amount of operation of said control lever, said second control valve including a movable member operatively connected to said control member of the pump for stopping said other flow of hydraulic fluid when said control member has been moved to said position corresponding to the discharge pressure of the pump, whereby the maximum discharge flow discharged by the pump can be restricted in conformity with the discharge pressure of the pump.

The present invention will be clearly understood from the description set forth hereinafter when considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing the arrangement of hydraulic motors in a hydraulic shovel;

FIG. 2 is a diagram of one example of the circuit for hydraulic fluid;

FIG. 3 is a schematic view of an example of the trench digging operation performed by simultaneously operating a number of hydraulic motors;

FIG. 4 is a schematic perspective view of the mechanism for controlling pumps for operating a hydraulic shovel according to this invention, with certain parts being shown in section;

FIG. 5 (1) is a sectional view of the hydraulic servomechanism used in the control device embodying this invention, said hydraulic servomechanism having a rotary valve shown in a transverse sectional view taken along the line H of FIG. 6;

FIG. 5 (2) is a transverse sectional view of the rotary valve of the servomechanism taken along the line II-II of FIG. 6;

FIG. 6 is a longitudinal sectional view of the rotary valve of the hydraulic servomechanism taken along the line VI-VI of FIG. 5;

FIGS. 7 to 12 are views in explanation of the operation of the hydraulic servomechanism used in the control device embodying this invention;

FIG. 13 is a diagram showing the PQ curves of the pump; and

FIG. 14 is a diagram showing variations in the characteristics curves of the two hydraulic pumps shown in FIG. 2.

A hydraulic shovel is generally provided with six hydraulic motors including a boom cylinder 1, arm cylinder 2, bucket cylinder 3, motor 4 for turning around the apparatus, motor 5 for moving the apparatus to the right, and motor 6 for moving the apparatus to the left as shown in FIG. 1. In order to increase operation efficiency, not less than two hydraulic motors are generally operated simultaneously. Hydraulic fluid under pressure is delivered to said hydraulic motors by two pumps 8 and 9 driven by a single prime mover 7 as shown in FIG. 2. The pump 8 supplies hydraulic fluid to the boom cylinder 1, bucket cylinder 3 and the motor 5 for moving the apparatus to the right, while the pump 9 supplies hydraulic fluid to the arm cylinder 2, motor 4 for turning around the apparatus, and motor 6 for moving the apparatus to the left. By operating triple valves 10 and 11, it is possible to operate any hydraulic motor as desired singly or in combination, for example, to operate the boom cylinder 1 and arm cylinder 2 simultaneously to move a bucket 12 in the direction of arrow 13 to perform a digging operation as shown in FIG. 3, In such case, it is required that the sum of the quantity of flow Q the pump 8 multiplied by its discharge pressure I and the quantity of flow Q of the pump 9 multiplied by its discharge pressure P or Q,-l Q,-P be maintained below the maximum output power of the prime mover 7. If, for example because the bucket 12 bites too much in the direction of arrow 14, the discharge pressure of the pumps is increased and the sum of output power of the two pumps exceeds the maximum output power of the prime mover, the number of revolutions of the prime mover will be reduced or the prime mover will stop rotating, thereby interfering with the smooth operation of the shovel.

The present invention provides a mechanism which permits controlling the motors 8 and 9 such that the output power of the prime mover 7 can be maintained near the IOOpercent level of its maximum output power when one or both of said pumps are loaded.

The invention will now be explained with reference to a preferred embodiment shown in the drawings. FIG. 4 shows the connection of an operation lever 15 for a position selector valve 10 and a control lever 17 of a hydraulic servomechanism 16 for effecting adjustments of the quantity of hydraulic fluid discharged by the pump 8 shown in FIG. 2. In FIG. 4, the pump 8 of the variable displacement type is an axial plunger pump having a swashplate. The prime mover 7 rotates through a gearing 18, a group of cylinders 20 firmly fixed to a shaft 19.

Pistons 21 built into said group of cylinders 20 move back and forth in one'stroke while sliding over the inclined surface of the swashplate 22 to draw hydraulic fluid by suction from a tank 23' and discharge the same into a line 24 for delivery to the hydraulic motors l, 3 and 5 through the triple valve 10. A cam 26 is provided on a rod 25 connecting one member of the triple valve 10 or a spool 10a, for example, to the operation lever 15. A roller 29 mounted at one end of a bellcrank 28 pivotally mounted on a fixed bracket 27 is urged by the biasing force of a spring 30 to press against said cam 26 at all times. The bellcrank 28 is connected at the other end to the control lever 17 of the hydraulic servomechanism 16 through a rod 31. It is to be noted that the operation lever 15 and the control lever 17 are associated with each other such that the control lever 17 is in its maximum displacement position when the operation lever 15 is in its maximum stroke position and the control lever 17 minimizes the angle of inclination of the swashplate 22 when the operation lever 15 is in its neutral position. FIG. shows the construction of the hydraulic servomechanism 16 adapted to vary the angle of inclination of the swashplate 22. The hydraulic fluid for operating the hydraulic servomechanism 16 is supplied by a trochoid pump 32, which is rotated by the rotary shaft 19 of the variable displacement pump 8, through a fluid passage 33 as shown in FIG. 4. A servo piston 34 has a pawl 35 which is firmly fixed thereto and maintained in engagement with an angle block 36 rotatably mounted at one end of the swashplate 22 for varying the angle of inclination of the swashplate 22 depending on the amount of movement of the servo piston 34. In FIG. 5 (1), the servo piston 34 is shown in a position in which the angle of inclination of the swashplate 22 is maximized. Covers 37 and 38 restrict the range of movement of the servo piston 34. A charge of hydraulic fluid is delivered to and withdrawn from chambers 39and 40 disposed on the left and right of the servo piston 34 through a rotary valve 41 operated by the control lever 17 and a pilot sleeve 43 moved by a follower lever 42. The rotary valve 41 comprises the combination of a valve body 44 and a follower sleeve 45 as shown in F IG. 6. The valve body 44 is rotated by the control lever 17 and the follower sleeve 45 is rotated by the follower lever 42 which is in engagement with the servo piston 34.

If the operation lever is returned to its neutral position from its maximum stroke position, the control lever 17 moves in the direction of arrow as shown in FIG. 7 to rotate the valve body 44 in the anticlockwise direction whereby a port 46 is opened between the valve body 44 and the follower sleeve 45 as shown in FIG. 7 (2) for discharging the quantity of hydraulic fluid in the chamber 40 disposed rightwardly of the servo piston 34 through a fluid passage 47, port 46 groove 48 in the valve body 44 and center opening 49. Since the chamber 39 disposed Ieftwardly of the servo piston 34 contains a quantity of hydraulic fluid delivered under pressure by the trochoid pump 32 through an oil passage 33, the servo piston 34 moves to the right in FIG. 7 (1). This causes the follower lever 42 to rotate the follower sleeve 45 in the anticlockwise direction to thereby close the port 46 and stop the movement of the servo piston 34. The rightward movement of the servo piston minimizes the angle of inclination of the swashplate 22, whereby the quantity of hydraulic fluid discharged by the pump 8 can be controlled to a minimum level. FIG. 8 shows the servomechanism 16 in this state. When the servomechanism is in this state, the pilot sleeve 43 is moved to the left by the other end of the follower lever 42 with respect to a pilot spool 50 to open ports 51 and 52, so that the fluid passage 33 maintains communication with a groove 54 in the valve body 44 through a fluid passage 53.

If the operation lever 15 is moved from its neutral position to right or left to actuate the position selector valve 10 while the servomechanism is in the state described above, the control lever 17 is moved by the cam 26 in the clockwise direction, so that a port 55 is opened between the groove 54 in the valve body 44 and the follower sleeve 45 as shown in FIG. 9 (2) for communicating the fluid passage 33 with the chamber 40 disposed rightwardly of the servo piston 34 through the fluid passage 47. Because of a difference in areas between the chambers 39 and 40 which are subjected to the pressure of hydraulic fluid, the servo piston 34 moves to the left in FIG. 8 (1), whereby the angle of inclination of the swashplate 22 is increased to increase the quantity of hydraulic fluid discharged by the pump 8. When the port 55 is closed by the follower sleeve 45 rotated in the clockwise direction by the follower lever 42, the servo piston 34 stops its movement. At this time, the pilot sleeve 43 also moves to the right to close the port 51 so that the servomechanism is restored to a state shown in FIG. 5. It will be appreciated that if the position selector valve 10 is actuated by the operation lever 15, the

control lever 17 coupled to the operation lever 15 is operated such that the angle of inclination of the swashplate is varied so that the quantity of hydraulic fluid discharged by the pump may be varied, with the quantity of fluid discharged by the pump being minimized when the operation level 15 is in its neutral position.

The mechanism of operation will now be explained in which the quantity of hydraulic fluid discharged by the pump is automatically restricted when the discharge pressure of the pump is increased so as to thereby reduce the output power of the pump. Referring to FIG. 5 again, the pilot spool 50 is arranged such that it is urged from the left by the biasing force of a spring 56 to move to the right and also urged from the right by the pressure of hydraulic fluid introduced through a fluid passage 57 to move to the left, so that when the pressures applied to the pilot spool from left and right balance the displacement of the spool 50 is proportional to the discharge pressure of the pump. The pilot spool50 in the state shown in FIG. 5 moves to the left as the discharge pressure of the pump is increased to thereby open a port 59 in the pilot sleeve 43 as shown in FIG. 10. Thus, the quantity of hydraulic fluid filling the chamber 40 rightwardly of the servo piston 34 is discharged through the port 59 so that the servo piston 34 moves to the right as seen in FIG. 10. If the discharge pressure is increased after the quantity of hydraulic fluid in the chamber 40 is discharged, the port 51 is opened and the port 59 is closed as shown in FIG. 12, so that a quantity of hydraulic pressure is introduced through the rotary valve 41 and port 55 into the chamber 40 disposed rightwardly of the servo piston 34 as seen in FIG. 12 (2). This causes the piston 34 to move to the left as seen in FIG. 12 (2). As the servo piston 34 moves to the left, the pilot sleeve 43 is caused by the follower lever 42 to return to its original position, with the result that the port 51 is closed again as shown in FIG. 11. It will be appreciated that as the discharge pressure of the pump acting on the pilot spool 50 is varied, the servo piston 34 is operated such that when the discharge pressure of the pump is increased the servo piston moves to the right to reduce the angle of inclination of the swashplate so as to thereby reduce the quantity of hydraulic fluid discharged by the pump, and when the discharge pressure is reduced the servo piston moves to the left so as to thereby increase the quantity of hydraulic fluid discharged by the pump.

The spool valve comprising the combination of the pilot spool 50 and the pilot sleeve 43 is mounted in series with the rotary valve 41 operated by the control lever 17 in the hydraulic servomechanism control circuit.

If the discharge pressure of the pump is increased when the angle of inclination of the swashplate is maximized and the quantity of hydraulic fluid discharged by the pump is maximized as shown in FIG. 10, then the port 59 in the pilot sleeve 43 is opened and the servo piston 34 moves to the right to reduce the angle of inclination of the swashplate 22 so as to thereby reduce the quantity of hydraulic fluid discharged by the pump. The servomechanism is in the state shown in FIG. 11 at this time. The magnitude of the stroke of rightward movement of the servo piston 34 is determined by an increase in the discharge pressure of the pump, and the control lever 17 remains in the state shown in FIG. 10. Therefore, the maximum quantity of hydraulic fluid discharged by the pump is after all limited by the pilot spool 50 in conformity with the discharge pressure of the pump, and no variation occurs in the quantity of hydraulic fluid discharged by the pump no matter how the control lever is moved in the range of its pivotal motion 11 shown in FIG. 11. If the discharge pressure of the pump is reduced when the servomechanism is in the state shown in FIG. 11 and the pilot spool 50 moves to the right as shown in FIG. 12 to open the port 51 in the pilot sleeve 43 as shown in FIG. 12 (1), then the fluid passage 33 communicates with the chamber 40 disposed rightwardly of the servo piston 34 through the rotary valve 41, so that the servo piston 34 moves to the left and the pilot sleeve 43 is caused by the follower lever 42 to return to its original position. The servo piston 34 stops its leftward movement when the port 51 is closed by the return movement of the pilot sleeve 43. In this way, the quantity of hydraulic fluid discharged by the pump can be increased in accordance with a reduction in the discharge pressure of the pump. The servo piston stops its movement when the port 51 is closed by the movement of the pilot sleeve 43 and the port 55 between the valve body 44 and the follower sleeve 45 is closed. Whether the port 51 is opened or closed depends on the position of the control lever 17 and hence the position of the operation lever 15. Accordingly, it will be appreciated that if the operation lever is moved to its operation position, then the servo piston 34 moves to the left and restricts the maximum angle of inclination of the swashplate 22 to thereby determine the maximum quantity of hydraulic fluid discharged by the pump at that time.

If a suitable spring constant is selected for the spring 56 and a suitable lever ratio is selected for the follower lever 42 in the mechanism just described, it is possible to impart desired characteristics to the relation between the discharge pressure P of the pump and the quantity of hydraulic fluid Q discharged by the pump.

The solid line a is a P-Q curve obtained in the present invention and broken lines are PQ curves at constant horsepower in FIG. 13. The curve a crosses the curves at constant horsepower, indicating that the horsepower required is reduced as the quantity of flow is reduced.

In the digging operation shown in FIG. 3, it is desired that various operations be combined with one another and performed at quick motions in order to increase operation efficiency. To attain the end, it is necessary to cause the pumps to deliver a maximum quantity of hydraulic fluid to the hydraulic motors to the maximum of the output power of the prime mover while regulating the maximum discharge pressure of the pumps, in view of the performance of the hydraulic motors.

The P-Q curve of FIG. 13 is obtained when the pump 8 delivers a charge of hydraulic fluid to the boom cylinder 1 and the pump 9 delivers a charge of hydraulic fluid to the arm cylinder 2. FIG. 14 (1) shows the P-Q curve for the pump 8 and FIG. 14 (2) shows the PQ curve for the pump 9.

Wen the bucket 12 moves in the direction of arrow 13, the output power of the pump 8 can be expressed by P XQ and the output power of the pump 9 by P XQ P represents a maximum discharge pressure set by a safety valve 60 of H6. 4. if the load in the direction of arrow 14 is increased and the pressure in the boom cylinder 1 is increased to P then the quantity of hydraulic fluid discharged by the pump 8 is reduced to Q The horsepower required for operating the pump 8 is lower when the output power of the pump 8 is P 'XQ than when the output power is P XQ as shown in F l0. l4 (2), so that the prime mover 7 operates at short of full output power and leaves a margin of horsepower. Therefore, if the margin of horsepower is utilized to operate the operation lever for the arm cylinder to a direction in which its stroke is maximized, it is then possible to move the PQ curve of the pump 9 to the right and deliver the quantity 0,, of hydraulic fluid discharged by the pump 9 to the arm cylinder 2. In a normal hydraulic shovelling operation, the discharge pressure P rises to a maximum discharge pressure P in a short time interval, so that he quantity Q of hydraulic fluid discharged by the pump 8 will be discharged through the safety valve 60 at the maximum discharge pressure. In such case, it is possible to increase 0,, to Q," by moving the operation lever to its neutral position to move the P-Q curve of the pump 8 to the left and reduce the horsepower required for operating the pump 8, so that the margin of horsepower can be utilized for the pump 9 to move the operation lever for the arm cylinder in a direction in which its stroke is maximized. If it had not been for the mechanism coupling the operation lever to the swashplate of the pump, an unnecessarily high power would be wasted on the pump 8. If the sum of output power of the two pumps exceeds the output power of the prime mover as a result of moving the operation lever too greatly in a direction in which its stroke is maximized, the number of revolution of the prime mover will be reduced. When the operator detects this phenomenon, he can return the lever slightly toward its neutral position, so that the P-Q curve can be moved to the left to redress the balance between the output power of the pumps and the output power of the prime mover.

The use of control device embodying the present invention as described above makes it possible to maintain the sum of output power of more than two pumps at the lOOpercent level of the output power of the prime mover by operating the operation lever during a shovelling operation, whereby amaximum quantity of hydraulic fluid can be delivered by each pump in conformity with its discharge pressure. This makes it possible to perform a shovelling operation at a high efficiency.

During the operation, the angle of inclination of the swashplate is minimized when the operation lever is moved past its neutral position for the purpose of switching the direction of supply hydraulic fluid to the hydraulic equipment by means of the position selector valve by moving the operation lever from one end to the other end. lf the operation lever is maintained in its neutral position during suspension of the operation, the angle of inclination of the swashplate will be minimized, making it possible to economize on the consumption of fuel during the idling of the prime mover and cause less wear and tear on the pumps.

We claim:

1. A device for controlling pumps for operating hydraulic motors comprising at least two variable displacement pumps operated by a single prime mover, valve means for each said pump disposed between the discharge side of the pump and the hydraulic motors for switching a charge of hydraulic fluid delivered to the hydraulic motors upon operation of an operation lever, and a hydraulic servomechanism for each said pump operated by a control lever coupled to said operation lever for moving a discharge flow control member of the pump, said operation lever for said valve means and said control lever for said hydraulic servomechanism being operatively connected to each other such that when the former is in its maximum stroke position the latter is in a position in which the discharge flow of the pump is maximized and when the former is in its neutral position the latter is in a position in which the discharge flow of the pump is minimized, said hydraulic servomechanism having a control circuit including a first control valve operated by said control lever for forming a flow of hydraulic fluid therethrough to move said discharge, flow control member of the pump to a position corresponding to the amount of operation of the control lever, and a second control valve responding to the discharge pressure of the pump for forming another flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the discharge pressure of the pump, said first control valve including a movable member operatively connected to said control member of the pump for stopping said flow of hydraulic fluid when said control member has been moved to said position corresponding to the amount of operation of said control lever, said second control valve including a movable member operatively connected to said control member of the pump for stopping said other flow of hydraulic fluid when said control member has been moved to said position corresponding to the discharge pressure of the pump, said movable member of said second control valve being operatively connected to said movable member of said first control valve such that the second control valve controls the discharge pressure of the pump in a range of the discharge flow of the pump below the maximum discharge flow as determined by the first control valve.

2. A device for controlling pumps as defined in claim 1 in which said hydraulic servomechanism includes a servo piston controlled by said control circuit for moving said discharge flow control member of the pump, said movable member of said first control valve and said movable member of said second control valve being connected to said servo piston through a follower lever.

member of the second control valve consists of a movable sleeve enclosing said valve spool for axial sliding motion relative to the same for opening and closing valve passages of the second control valve, and said follower lever is connected to said rotary sleeve to turn about the axis of said rotary sleeve and pivotally connected to said servo piston at one end and to said movable sleeve at the other end. 

1. A device for controlling pumps for operating hydraulic motors comprising at least two variable displacement pumps operated by a single prime mover, valve means for each said pump disposed between the discharge side of the pump and the hydraulic motors for switching a charge of hydraulic fluid delivered to the hydraulic motors upon operation of an operation lever, and a hydraulic servomechanism for each said pump operated by a control lever coupled to said operation lever for moving a discharge flow control member of the pump, said operation lever for said valve means and said control lever for said hydraulic servomechanism being operatively connected to each other such that when the former is in its maximum stroke position the latter is in a position in which the discharge flow of the pump is maximized and when the former is in its neutral position the latter is in a position in which the discharge flow of the pump is minimized, said hydraulic servomechanism having a control circuit including a first control valve operated by said control lever for forming a flow of hydraulic fluid therethrough to move said discharge, flow control member of the pump to a position corresponding to the amount of operation of the control lever, and a second control valve responding to the discharge pressure of the pump for forming another flow of hydraulic fluid therethrough to move said discharge flow control member of the pump to a position corresponding to the discharge pressure of the pump, said first control valve including a movable member operatively connected to said control member of the pump for stopping said flow of hydraulic fluid when said control member has been moved to said position corresponding to the amount of operation of said control lever, said second control valve including a movable member operatively connected to said control member of the pump for stopping said other flow of hydraulic fluid when said control member has been moved to said position corresponding to the discharge pressure of the pump, said movable member of said second control valve being operatively connected to said movable member of said first control valve such that the second control valve controls the discharge pressure of the pump in a range of the discharge flow of the pump below the maximum discharge flow as determined by the first control valve.
 2. A device for controlling pumps as defined in claim 1 in which said hydraulic servomechanism includes a servo piston controlled by said control circuit for moving said discharge flow control member of the pump, said movable member of said first control valve and said movable member of said second control valve being connected to said servo piston through a follower lever.
 3. A device for controlling pumps as defined in claim 2 in which said first control valve comprises a rotary valve element having valve passages and operated by said control lever, said movable member of the first control valve consists of a rotary sleeve enclosing said rotary valve element to rotate relative to the same and having ports adapted to open and close said valve passages, said second control valve comprises a valve spool urged by a spring at one end therEof and subjected to the discharge pressure of the pump at the other end, said movable member of the second control valve consists of a movable sleeve enclosing said valve spool for axial sliding motion relative to the same for opening and closing valve passages of the second control valve, and said follower lever is connected to said rotary sleeve to turn about the axis of said rotary sleeve and pivotally connected to said servo piston at one end and to said movable sleeve at the other end. 